libext2fs: search all possible blocks in implied_cluster_alloc()

libext2fs: search all possible blocks in implied_cluster_alloc()

implied_cluster_alloc() is written such that if the the user passes in
a logical block that is the zeroth block in a logical cluster (lblk %
cluster_ratio == 0), then it will assume that there is no physical
cluster mapped to any other part of the logical cluster.

This is not true if we happen to be allocating logical blocks in
reverse order.  Therefore, search the whole cluster, except for the
lblk that we passed in.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>